In an internal combustion engine mounted in an automobile etc, particularly in a lean combustion type internal combustion engine that tends to have a small exothermic quantity as in a diesel engine, there is known a technology of providing a combustion heater together for the purpose of enhancing a performance of a heating device for a room and speeding up a warm-up of the internal combustion engine at a cold time.
The combustion heater described above includes, for example, a combustion chamber independent of the internal combustion engine, a water passageway (which will hereinafter be called an [intra-heater cooling water passageway]) formed so as to surround this combustion chamber, an engine cooling water introduction pipe for leading the engine cooling water into the intra-heater cooling water passageway from a water jacket of the internal combustion engine, and an engine cooling water discharge pipe for leading the engine cooling water to the water jacket from the intra-heater cooling water passageway.
In the thus constructed combustion heater, if a temperature of the engine cooling water is low as when the internal combustion engine is in a cold state, the cooling water in the water jacket is led to the intra-heater cooling water passageway, and a part of engine fuel is combusted in a combustion chamber. The engine cooling water led to the intra-heater cooling water passageway receives heat generated when the fuel is burnt in the combustion chamber, thereby raising a temperature of the engine cooling water. The engine cooling water having the thus raised temperature is led to the water jacket from the intra-heater cooling water passageway, and the heat of the engine cooling water is transferred to the internal combustion engine.
As a result, the internal combustion engine can be warmed up at an early stage. Note that when a heater core for heating a car room is disposed on a circulation route of the engine cooling water, the heat of the engine cooling water having the raised temperature in the combustion heater can be also transferred to the air for heating in the heater core. Hence, in the internal combustion engine having the combustion heater, it is possible to speed up the warm-up of the internal combustion engine at the cold time and to enhance the performance of the car room heating device.
On the other hand, the combustion heater performs the combustion by utilizing the fuel of the internal combustion engine as described above, and hence it is desirable that the combustion gas discharged from the combustion heater be purified as in the case of an exhaust gas of the internal combustion engine.
In response to such a request, there is proposed a vehicle heating device including the combustion heater as disclosed in Japanese Patent Application Laying-Open Publication No. 60-78819. The vehicle heating device including the combustion heater disclosed in the above Publication is constructed so that the combustion gas discharged from the combustion heater is led to a portion, disposed more upstream than an exhaust gas purifying device, of an engine exhaust pipe. Accordingly, in the vehicle heating device including the combustion heater, the combustion gas from the combustion heater is purified together with the exhaust gas of the internal combustion engine by the exhaust gas purifying device.
The known exhaust gas purifying device is a catalyst converter carrying a three-way catalyst, an occlusion reduction type lean NOx catalyst or a selective reduction type lean NOx catalyst and so on. In this type of catalyst converter is, however, when a temperature of the catalyst of this catalyst converter is uniformly equal to or higher than a predetermined activation temperature, the catalyst is activated, and harmful gas components contained in the exhaust gas can be purged of. Therefore, the catalyst converter is incapable of sufficiently purging the exhaust gas of the harmful gas components when the catalyst temperature of the catalyst converter is lower than the activation temperature.
By the way, in the lean combustion type internal combustion engine, a quantity of the fuel supplied for the combustion is small, a combustion temperature therefore tends to decrease, and an exhaust temperature tends to decrease corresponding thereto. Especially when the lean combustion type internal combustion engine is in a low-load operation state, the fuel supplied for the combustion becomes be far smaller, and hence the exhaust temperature tends to become by far lower. Accordingly, when the lean combustion type internal combustion engine is in the low-load operation state, there might be a case where the catalyst temperature of the catalyst converter is hard to be kept equal to or higher than the activation temperature only by the exhaust gas of the internal combustion engine.
By contrast, when the lean combustion type internal combustion engine is in a low-load operation area, as in the case of the vehicle heating device including the combustion heater described above, a method is such that the combustion gas discharged from the combustion heater is flowed into the exhaust gas purifying device, and a temperature of the catalyst converter is raised by utilizing the heat of the combustion gas.
In the vehicle heating device including the combustion heater described above, however, the combustion gas discharged from the combustion heater is a gas after undergoing a heat exchange with the engine cooling water and therefore has a less heat quantity.
Further, in the case where the temperature of the engine cooling water is low as when the internal combustion engine is in the cold state, upon an execution of the heat exchange between the combustion gas and the engine cooling water, it follows that a comparatively large quantity of heat is transferred to the engine cooling water from the combustion gas, and there might be a possibility where the temperature of the combustion gas discharged from the combustion heater gets extremely low. Therefore, even when introducing into the exhaust gas purifying device the combustion gas after undergoing the heat exchange with the engine cooling water in the combustion heater, it is difficult to promptly activate the catalyst converter.